Desert Crops Workshop covers lettuce leaf drop, perchlorate

Levels of rocket fuel component perchlorate have dropped dramatically over the past few months in the Las Vegas Wash that drains into the Colorado River, giving rise to optimism that levels of the contaminant linked to thyroid dysfunction and hormone imbalance may be declining in the lower Colorado River.

The Colorado River is a major drinking water source for more than 20 million people in California, Nevada and Arizona and irrigation water for 1.4 million acres of farmland.

Detection of perchlorate in the water raised concerns in all three states. High levels of the rocket fuel component have been identified as possibly harmful to infants and pregnant women. However, it has not been identified as a carcinogen.

Charles Sanchez, director of the University of Arizona Yuma Agriculture Station, told the Desert Crops Workshop recently in Yuma, Ariz., that he has detected some decline in perchlorate levels at the dam water diversion point. However, he wants to do more sampling to see if it will be consistent. He added he has measured declines in perchlorate levels in the past where irrigation water is diverted for California and Arizona farmers only to see them go up again later.

Nevertheless, he is “very optimistic” cleanup efforts at an old rocket fuel plant near Henderson, Nev., may soon lead to significantly reduced levels of perchlorate in the river and in crops grown with river irrigation water.

The 15th Desert Crops Workshop was sponsored by the Cooperative Extension services at the University of California and University of Arizona and Western Farm Press.

Imperial Dam is the diversion point for 4.2 million acre feet of water to irrigate annually 1.4 million acres of farm land along the Colorado River in Southwestern Arizona and Southeastern California. Perchlorate has been found in leaf lettuce produced during the winter in the desert valleys along the Colorado but at levels far below what is considered a health hazard, said Sanchez. About 70 percent of winter lettuce and other vegetables consumed in the U.S. are produced in Imperial Valley and in the Yuma area.

Dropping fast

“Perchlorate levels are dropping pretty fast where the Las Vegas Wash dumps into Lake Mead. Perchlorate levels just below Lake Mead are also dropping,” said Sanchez.

He hopes to see similar drops at Imperial Dam. The reason he is optimistic now that the contaminant levels will go down along the lower Colorado River and continue to decline is due to the stepped up efforts of Kerr-McGee Corp., operator of the rocket fuel plant that leaked perchlorate into the groundwater, to clean the contaminant out of the groundwater. Kerr-McGee is using a new clean-up technique that is cleaning up more contaminated water quicker than the old method that was used until last May.

However, what are harmful levels remains a big debate. The federal government has not set a safety standard, although California has set 6 parts per billion in drinking water as the maximum level considered safe. Arizona has set 14 ppb as a nonbinding “health based guidance level.” The National Academy of Science is expected to announce this month what it considers a safe level. A federal proposal calls for making it 1 ppb, but that is being challenged.

Levels Sanchez has measured in Colorado River water and desert-grown lettuce have been below the California drinking water standard of 6 ppb.

The California standard assumes that if a person drinks two liters of water per day he or she will consume six micrograms of perchlorate per day. Sanchez said perchlorate levels in lettuce he has quantified are far below that exposure level.

Low in vegetables

The highest levels of perchlorate have been found in the outer lettuce leaves, which are often discarded in the field. For other vegetable crops grown in the winter in the desert, Sanchez has either measured extremely low levels of perchlorate or no detectable levels. He believes this rules out any cumulative effect of ingesting high levels of perchlorate in consuming different vegetables.

Perchlorate does accumulate in the soil with the irrigation water and is taken up by lettuce. However, Sanchez said it can be quickly removed from the root zone. “Two irrigations (with uncontaminated water) completely flushes out the perchlorate,” he said. “Once the river is cleaned up, I think we can mitigate the soils very quickly,” said Sanchez.

“I am very optimistic we will weather this and get away from the problem very quickly,” said Sanchez.

Colorado River has been the focus of the perchlorate contamination issue centered on rocket fuel and munitions manufacturing. After perchlorate was detected in the Colorado River, other states began testing water supplies. Perchlorate has been found in water supplies in 22 states across the nation. Most have been associated with rocket fuel or munitions plants, but Sanchez said there are also natural sources of perchlorate. It has been detected in fertilizer from Chile and it is found in the water of the Ogallala Aquifer in Texas. It also has been reported in playas in the West.

Although the perchlorate contamination issue has focused on desert-grown lettuce, Sanchez has tested lettuce grown in the central and coastal valleys of California, the Mesilla Valley of New Mexico, Southern Colorado and other areas.

National sampling

“We collected 440 samples throughout the nation; 254 conventionally grown and 176 organically grown,” he said. Eighteen percent of the conventionally grown lettuce had quantifiable levels of perchlorate. However, 30 percent of the organic lettuce had quantifiable levels of perchlorate.

He would not speculate on why more perchlorate was detected in organic lettuce than conventional lettuce. “It may be some amendment that is used in organic lettuce. It is highly unlikely all organic lettuce was farmed near aerospace or defense related industry plants,” said Sanchez. Perchlorate has also been detected in milk.

However, the Food and Drug Administration has recommended that no one should change their diet. FDA recommends milk and vegetables as important parts of a healthy diet.

To put these most recent milk findings in perspective, a person would drown in milk, for example, before perchlorate could pose a health risk. The highest level detected in some samples of milk was 11.3 parts per billion. According to Michael Payne, a toxicologist at the University of California at Davis, a person would have to drink 2,000 eight-ounce glasses of milk a day at least, for weeks or months, before there could be any risk of adverse health effects from perchlorate.